Since the oxidation-reduction potential of Ce4+ and Ce3+ is small at about 1.6 V and the reaction represented by the following formula proceeds reversibly, cerium oxide has an oxygen storage capacity (OSC), and is used as a co-catalyst or catalyst support of automotive three-way catalysts.CeO2CeO2-x+X/2O2 (X=0 to 0.5)
However, the OSC of pure cerium oxide is well known to be extremely low at about X=0.005.
Thus, in order to improve this, there have been numerous reports describing (1) improving the heat resistance of the specific surface area of cerium oxide, and (2) improving the OSC by inserting Zr4+ having a small ionic radius into a Ce backbone to alleviate the increase in volume during the above-mentioned reaction, by putting zirconium oxide into a solid solution with cerium oxide.
On the other hand, in the case of using pure cerium oxide as a co-catalyst or catalyst support of an automotive three-way catalyst, the dispersibility thereof is extremely good in the case of loading with a precious metal, and particularly platinum. Namely, it is a known fact that the cerium oxide has the superior characteristic of being able to inhibit aggregation of platinum particles at high temperatures.
Therefore, there has recently been a demand for a cerium oxide-zirconium oxide-based mixed oxide having both of these characteristics.
Japanese Examined Patent Publication No. H06-74145 describes a “composition mainly consisting of ceric oxide having a stabilized specific surface area, comprising ceric oxide and at least one type of additive at a ratio of 1 to 20% by weight, wherein the additive is an oxide of one or more types of other metal elements A selected from the group consisting of silicon, zirconium and thorium”.
However, only the heat resistance of ceric oxide containing 2.5% zirconia is described in the examples.
In addition, Japanese Patent No. 3623517 describes a “composition composed of cerium oxide and zirconium oxide having an atomic ratio of cerium/zirconium of at least 1 and demonstrating a specific surface area of at least 35 m2/g after being calcined for 6 hours at 900° C. and an oxygen storage capacity of at least 1.5 ml/g O2 at 400° C.”.
However, there is no description regarding the dispersibility of platinum.
On the other hand, Japanese Patent Application Publication No. 2002-177781 describes an “exhaust gas purification catalyst comprising a mixed oxide (B) comprised of cerium oxide loaded onto a solid solution oxide (A) containing zirconium and cerium”.
However, this catalyst has a unique structure in which cerium oxide is loaded around a solid solution oxide (A) containing zirconium and cerium, and what is more, there are no descriptions whatsoever regarding the OSC and platinum dispersibility thereof.
Moreover, Japanese Patent Application Publication No. 2005-314134 describes “metal oxide particles having a core portion containing a comparatively large amount of a ceria-zirconia solid solution and a shell portion containing a comparatively large amount of a second metal oxide”, and a “production process of metal oxide particles having a core portion containing a comparatively large amount of a ceria-zirconia solid solution and a shell portion containing a comparatively large amount of a second metal oxide that provides a sol containing colloidal particles of a ceria-zirconia solid solution and colloidal particles of a second metal oxide each having different isoelectric points, comprising making the pH of the sol closer to the isoelectric point of the colloidal particles of the ceria-zirconia solid solution than the isoelectric point of the colloidal particles of the second metal oxide, aggregating the colloidal particles of the ceria-zirconia solid solution, making the pH of the sol closer to the isoelectric point of the colloidal particles of the second metal oxide than the isoelectric point of the colloidal particles of the ceria-zirconia solid solution, aggregating the colloidal particles of the second metal oxide around the aggregated colloidal particles of the ceria-zirconia solid solution, and drying and firing the resulting aggregate”.
However, although there are descriptions relating to OSC and platinum dispersibility, the shell portion and core portion are characterized by both being in the form of ceria-zirconia solid solutions, and the production process is extremely unique.